Learning Goals:

  1. Explain the logic of experiments and describe how randomization can be used to solve the fundamental problem of causal inference

Review of Enos Trains Study:

  1. What was the hypothesis that Enos wanted to test?
  2. If Enos already knew that people living in areas with higher concentrations of immigrants were more hostile to immigration, why did he decide to run an experiment in the first place?
  3. How did Enos’ experiment work? What two groups did he compare? In what ways were these two groups different? In what ways were they the same?

Logic of Randomized Experiments

A Silly Example


Suppose you develop a new weight lose drug and want to demonstrate its effectiveness. You put an advertisement in the local newspaper asking for volunteers who want to try the drug.

A bunch of people respond to your ad. You invite them all to your office, and tell them about how your wonderful new drug will help them lose weight. You then weigh all of the volunteers (to establish a baseline measurement), and then send them home with instructions on how to take the drug.

Two weeks later, you invite them back to your office for another weighing. You are delighted to discover that the average person lost 5 kg.

(Besides being illegal), what are some problems with this research design? And what would you do differently?

Definitions
  • The treatment group (T): people who get the drug
  • The control group (C): people who do not get the drug, but get a placebo instead
  • Randomization: a way to assignment people to C and T such that, on average, the people in C are similar in their personal characteristics to the people in T
  • Blinding: people do not know which group - C or T - they are in


Key Idea: because the people in C are, on average, the same as the people in T, we have removed compositional (selection) effects through randomization. We can thus estimate the causal effect of treatment.


Experimenting with Repeated Contact

Suppose your friend sees Enos’ results and says:

“Nice experiment, but he’s just documenting a temporary reaction to the unexpected appearance of Latinos in all-white suburbs. Over time, however, people are going to become more comfortable with diversity. For example, cities have historically been magnets for immigration, and the people living there seem to have no problem with diversity.”

What do you think about this argument? What about the “evidence” your friend cites?

Think-pair share: How would you modify Enos’ experiment to test the hypothesis that prolonged contact can moderate the negative influence of exposure to immigration?

Results: